1. Field of the Invention
The present invention relates to a sheet stacking apparatus, a method of controlling the sheet stacking apparatus, and a storage medium.
2. Description of the Related Art
In a conventional sheet stacking apparatus configured to discharge sheets to a sheet stacking unit that can be moved up and down, an image is printed on a sheet, and the sheet on which the image is printed is discharged from a sheet discharging port to the sheet stacking unit. As illustrated in FIG. 12A, the sheet stacking unit is moved down according to the amount of sheets stacked on the sheet stacking unit such that the uppermost surface of the sheets stacked on the sheet stacking unit is located near the sheet discharging port, whereby sheets discharged from the sheet discharging port can be stacked stably.
If an obstacle that prevents the sheet stacking unit from being moved down is placed under the sheet stacking unit as illustrated in FIG. 12B, the sheet stacking unit hits the obstacle while the sheet stacking unit is moved down. If the sheet stacking unit is forced to be moved down even after the sheet stacking unit hits the obstacle, a load is applied to a driving unit configured to move down the sheet stacking unit, and this may damage the sheet stacking unit and the driving unit. To overcome this problem, a method is discussed in which, in a case where an operation to move down a sheet stacking unit is prevented by an obstacle, a sheet discharging operation and the operation to move down the sheet stacking unit are stopped, and a warning is displayed (refer to Japanese Patent Application Laid-Open No. 2001-226022).
Meanwhile, when sheets are stacked on one sheet stacking unit, the sheet bundle may become so high that the sheet bundle may collapse. Japanese Patent Application Laid-Open No. 2012-12182 discusses a solution to this problem. Japanese Patent Application Laid-Open No. 2012-12182 discusses including two sheet stacking units so that the height of a sheet bundle stacked on the sheet discharging trays is not likely to become excessively high to prevent the sheet bundle from collapsing. Then, when excessive sheets are stacked on one of the sheet stacking units, the output is switched to the other one of the sheet stacking units to maintain the stacking capacity without decreasing the overall amount of stacks.
A space under a sheet stacking unit is convenient for storing consumables such as sheets and apparatus manuals. Further, the location of the space enables easy access to stored consumables and manuals. Furthermore, there are users considering that a print job that causes the sheet stacking unit to be moved down to the lower limit is not frequently executed. Thus, there are cases where an object is intentionally placed under the sheet stacking unit by the users.
In the case where a warning to the user is displayed when an obstacle preventing the sheet stacking unit from being moved down is detected as discussed in Japanese Patent Application Laid-Open No. 2001-226022, the user attempts to remove the obstacle in response to the warning. However, since the obstacle prevents the sheet discharging tray from being moved down, the sheet stacking unit and the obstacle are in contact with each other.
Further, in the case where there are multiple sheet stacking units as discussed in Japanese Patent Application Laid-Open No. 2012-12182, it is desired to switch the output to the other one of the sheet discharging trays to continue the output. However, if the sheet stacking unit is moved when the sheet stacking unit is in contact with the obstacle, the obstacle may be damaged, or the sheet discharging tray may be damaged to result in breakdown of the apparatus.